1. Field of the Invention
The present invention relates to a composition, and more particularly to a composition, and more particularly to a releasing agent which is very suitable for treatment of a released sheet or a pressure sensitive adhesive tape or sheet to impart a good releasing property thereto, wherein the pressure sensitive adhesive tape or sheet has a base comprising, a flexible film or sheet consisting of paper, non-woven fabric, cloth, cellulose acetate, regenerated cellulose, polyethylene terephthalate, soft polyvinyl chloride and so on.
2. Description of the Prior Art
A pressure sensitive adhesive tape or sheet is generally wound in a roll form in which an adhesive layer is piled up on an adjacent back surface of a tape or sheet base material or is protected with a released sheet. The adhesive layer is thus protected for a time, however, it should have a good releasing property while in use so that the roll-shapedly wound pressure sensitive adhesive tape is easily released from the roll form or the released sheet with a relatively weak force. According to a known method to improve the property of a parting surface of the base material or the released sheet, the parting surface is treated with a silicone coating. However, the known pressure sensitive adhesive tape having the silicone coating has the following diadvantages:
(a) The silicone coating must be printed and hardened on the parting surface at a relatively high temperature of 130.degree. to 180.degree. C., which causes a dimensional fluctuation such as elongation, shrinkage, bending or the like, or a material deterioration such as brittlement in the base film.
(b) The silicone coating is formed by polymerization of a low molecular weight compound and unhardened compound is liable to remain therein. Because these low molecular weight materials are transferred to the pressure sensitive adhesive layer, the adhesion force is lowered and an object with the adhesive layer adhering thereon is contaminated.
(c) There is a case that an adherence between a certain kind of base material and a releasing agent is insufficient. This causes a transference of the releasing agent and a contamination of the object.
(d) An adhesion force between the pressure sensitive adhesive layer and this releasing agent surface is too weak due to the good releasing property. As the result, the adhesive of the tape in one path can not sufficiently adhere to the back surface of the tape in another or adjacent inner path so that both are easy to separate from each other.
In order to overcome the above disadvantages, organic releasing agents has been studied. The known organic releasing agents are polyethylene, polypropylene, vinylidene chloride, tetrafluoroethylene, amino aldehyde, shellac, wax paraffins, chromium chloride stearate, a combination of chromium stearate and ethyl cellulose or a solvent, alkyl acid phosphate, a combination of titanium dioxide and nitrocellulose, fluorocarbon, perfluorocarbonoxylicamide, a clay blend of sodium alginate, sodium hydroxide and sodium hydrosilicate, a blend of polyvinyl butyral, hardened castor oil and stearic acid, a copolymer of vinyl pyrrolidone and vinyl stearate, a copolymer of vinyl alcohol and vinyl stearate, a copolymer of vinyl stearate, maleic acid anhydride and vinyl acetate, a copolymer of partial higher fatty acid ester of polyvinyl alcohol and higher alkyl vinyl ester or ether, higher alkylamine salts, polyvinyloctadecylurethane, a copolymer of acrylic acid and octadecyl acrylic acid, a copolymer of polyvinyl alcohol and N-octadecyl carbamate, a copolymer of polyvinyl alcohol and N-dihydroperfluoroalkylisocyanate, a copolymer of carbamic acid and polyvinyl alcohol, and so on. Effective releasing agent among these organic compounds is a polymer having carbon atoms of more than 13, particularly of 18 (octadecyl or stearyl group). For example, Japanese Patent Publication No. 5382/1961 discloses a four-component copolymer of octadecyl acrylic acid ester, acrylic acid, acrylonitrile and methyl acrylate. Japanese Patent Publication No. 9599/1969 discloses a binary copolymer of stearyl methacrylate and acrylonitrile. U.S. Pat. No. 2607711 uses a copolymer of octadecyl acrylate and arcylic acid. Japanese Patent Publication No. 8903/1965 uses a copolymer of octadecyl vinylether, acrylonitrile, maleic acid anhydride and vinyl acetate. Japanese Patent Publication Nos. 17661/1965, 2894/1967 and 5006/1968 disclose releasing agents which are produced by reacting alkylimine with higher alkyl isocyanate and higher alkyl carboxylic acid. Further, Japanese Patent Publication No. 21855/1968 discloses N-long chain alkyl (metha) acrylic acid amide.
The use of the releasing agent having the long chain alkyl group can admirably overcome some of the above-described disadvantages of the known silicone-type releasing agent. That is, as to the disadvantages (a) and (b), a polymer already produced by polymerization has only to be coated and dried on a base material as a plastic film. Accordingly, the high temperature printing is not required to form a releasing agent layer and the transferring and contamination by the lower molecular weight compound decreaser. The disadvantage (c) can be avoided because on the synthesis of the releasing agent monomers having various polarities can be easily introduced. The disadvantage (d) can be overcome by itself in such a way that an ordinary releasing strength of the organic parting agent is imparted to the releasing agent having the long chain alkyl group. This is because the adhesion force of the adhesive layer to the organic parting agent layer of the base material is not too weak different from the silicone-type releasing agent and therefore it has a suitable built-up beam property and readhesion strength. In this case, the users must take care in general that the adhesion force is too strong to peel off the adhesive layer again from the parting agent layer.
Thus, it has been supposed that a releasing agent suitable for the pressure sensitive adhesive tape having the base (plastic film) was finished by the use of the releasing agent containing the above higher alkyl group. However, there are important problems to be still avoided. The largest defect of the releasing agent is in an aging of the coating property when it is coated on the base material. It is supposed that the aging is due to the deterioration of the releasing property of the releasing agent itself and to the poor adherence between the releasing agent and the base material. The former is a phenomenon which is often observed on the pressure sensitive adhesive tape having the organic releasing agent layer. For example, the tape is wound off in the beginning at a velocity of 300 mm/min and with a force of 100 to 300 g per 5 cm width, however, in some cases it shows a winding-off force of 600 to 2,000 g or of more than 600 to 2,000 g per 5 cm width after a long time has passed. The reason of the aging of the adhering force at the interfacial surface between the releasing agent and the adhesive is still not understood. In this connection, it is generally said that a surface having a lower energy is formed in the case a density of methyl groups thereat is higher, and that a silicone-type releasing agent containing predominantly methyl silicone shows a very effective releasing property. Also the organic parting agent having the long chain alkyl group introduced can have such a lower energy surface, however, it can but form a very brittle releasing agent layer as it is and the transferring to the adhesive layer occurs. To prevent this, a high crystalline copolymer of the organic releasing agent and acrylonitrile or the like having, or an addition product consisting of the organic releasing agent and another polymer is used as a conventional organic releasing agent. When a releasing agent is coated on a base material such as regenerated cellulose having a relatively high polarity, functional groups having a high polarity are introduced into a part of the releasing agent (polymer), so that the adhering force to the base material is improved and the orientation regulatity of the long chain alkyl group is increased to promote the parting effect by increasing density of methyl groups at the surface. However, in the organic releasing agent produced by the conventional technique, it is impossible to remove the aging of the releasing property of the above-mentioned agent itself.
The inventor has well studied about this problem and found out the following fact. That is, the releasing property of the organic parting agent containing the long chain alkyl group changes largely at about 45.degree. C., which has a relation to the melting point of the oriented crystal of the long chain alkyl group. From this fact, the inventor has come to a conclusion that it is necessary to increase the density of the cross-linkages between the molecules of the releasing agent in order to prevent the change of the releasing property also at a temperature above the melting point of the long chain alkyl group segment. The conventional organic releasing agent has the disadvantage that the releasing property shows an aging, however, this has been overcome by the selected combination of the releasing agent and the adhesive, and the limitation of the condition of preservation or use, and so on. However, these measures can not bring about a fundamental solution of the disadvantage. In other words, the blocking of the adhesive tape due to the aging of the releasing property, the transferring of the releasing agent or the like is not avoided and the product can not be often provided for use. There is a known technique that cross-linking between the molecules of resin impart a heat resisting property thereto. However, since there was no clear explanation about the aging of the releasing property of the conventional organic releasing agent, a measure to prevent the aging by the cross-linking of the releasing agent can not almost found out. The inventor has studied the cross-linking of the releasing agent and confirmed that the conventional cross-linding has the following defects:
(a) According to the conventional cross-linking technique, for example, a condensation reaction of carboxylic group with hydroxyl group, an addition reaction of hydroxyl group with isocyanate, and the like, polar groups such as ester group, urethane group and so on are introduced into the molecule of the releasing agent. This results in an increased adhering force between the agent and the adhesive, and in a severe limitation of the base material.
(b) A significant heat or time are required for completing the reaction for the cross-linking. This causes a remarkable deterioration of the productivity of the product, and of the dimension and material of the base. Also in the case that unreacted substances still remain, the aging is further increased.